High-voltage electrical insulator having a predetermined surface conductance

ABSTRACT

A high-voltage electrical insulator of synthetic plastics material having a predetermined electrical conductance to inhibit discharges of the order of 7 milliamperes and less but to permit larger current discharges. Preferably said conductance is provided by surface conductance and preferably it is discharges of 5 milliamperes and less which are inhibited.

United States Patent Appl. No. Filed Patented Assignee HIGH-VOLTAGEELECTRICAL INSULATOR HAVING A PREDETERMINED SURFACE CONDUCTANCE 3Claims, No Drawings U.S. Cl .1 174/140 C, 174/209 lnt.Cl ..l-l0lb 17/42,HOlb 17/64 Field 01' Search 174/137. 1, 137.2, 138 C, 140, 140.1, 140C,141 C, 209, 210, 211

[56] References Cited UNITED STATES PATENTS 1,661,823 3/1928 Hawley174/140 CX 1,735,560 11/1929 Austin l74/140C 2,947,801 8/1960 Doolittle174/140 CX 3,243,505 3/1966 Clark 174/140 C 3,368,026 2/1968 Vince174/140 C 3,416,033 12/1968 Hoover et a1. ..1.74/140(.1 UX FOREIGNPATENTS 1,021,081 2/1966 Great Britain 174/21 1 1,068,219 5/1967 GreatBritain.... 174/140 C Primary Examiner Laramie E. Askin AuomeySommers &Young ABSTRACT: A high-voltage electrical insulator of syntheticplastics material having a predetermined electrical conductance toinhibit discharges of the order of 7 milliamperes and less but to permitlarger current discharges. Preferably said conductance is provided bysurface conductance and preferably it is discharges of 5 milliamperesand less which are inhibited.

HIGH-VOLTAGE ELECTRICAL INSULATOR HAVING A PREDE'I'ERMINED SURFACECONDUCTANCE This invention relates to high voltage electrical insulatorssuitable for outdoor use. Such insulators as widely used today are ofglazed porcelain and are perfectly satisfactory in operation. However,recent interest has been directed to the use of plastics materials forhigh-voltage insulators, for example, glass-fiber reinforced plasticsmaterials, which have certain advantages over glazed porcelain.

The use of high-voltage plastics insulators outdoors is at the presenttime seriously restricted by the phenomena known as tracking, i.e,carbonization of the surface of the insulator by surface electricaldischarges, which results in a permanent degradation of the insulatingproperties of the insulator. It is the object of this invention toprevent such tracking.

The invention is based on the discovery that if surface electricaldischarges of the order of 7 milliamperes and less are inhibited thereis no significant tracking; hence higher magnitude surface electricaldischarges can be permitted. Accordingly the invention provides aninsulator of synthetic plastics material having a continuous coating onits outer surface to produce a surface such that the plastics insulatorhas a predetermined electrical conductance so as to inhibit dischargesof the order of 7 milliamperes and less.

Preferably said electrical conductance is such as to inhibit dischargesof milliamperes and less but to permit higher current discharges.

Surface electric discharge of an insulator is caused by the rapiddrying-out of wet pollution on its surface by the leakage currentflowing through the pollution. In the case where, in accordance with theinvention, surface conductance is provided on the insulator it can. beshown that surface discharges will not occur unless the current throughthe pollution exceeds a limiting value according to the followingexpression:

V E 5 3T3 i,

where:

V/x is the voltage stress (volts per centimeter) at which the surfacebreaks down; this means that surface discharge cannot take place at alower stress,

E is the voltage across the insulator,

L is the minimum distance along the surface of the insulator between itsends,

i, is the minimum leakage current through the pollution at whichdischarge is to be permitted,

i, is the current that normally flows by reason of the surfaceconductance of the insulator when the insulator is dry.

Since in accordance with the invention the value of i, will be given bya value of the order of 7 milliamperes the surface conductance can bedetermined by applying Ohms law (K==l/E) to the calculated value of g.

If for example the critical value for War is l7.5 kv./cm, E is 35 kv.,(peak value of 25 kv. AC supply) L is 1 meter and p is greater than 5ma., then the surface conductance provided in the insulator will have toprovide a leakage current 3 of about A. and the conductance would beabout .000285p. mho.

If the surface conductance of the insulator increases (i.e. theresistivity decreases) with increasing voltage stress, for example, byusing known materials in the coating whose ohmic resistance decreaseswith an increase in voltage stress, then the steady state leakage can bereduced. Thus if the surface resistivity for a voltage stress E/L is 10times greater than that at the breakdown stress V/x then the steadystate current leakage through the dry insulator can be reduced say fromI00 to l0p.A. The surface conductance of the insulator may be providedby a coating of flexibilized resin intimately mixed with antimony dopedtin oxide powder, the coating being cured on the insulator.

We claim:

1. A high-voltage insulator comprising a synthetic plastic bod and acontinuous coatin of a conductive material on the su ace thereof, saidcoating avmg a thickness and berng of a material to inhibit discharge ofelectrical currents therethrough below about 7 mA while passing currentsabove said 7 mA.

2. An insulator in accordance with claim 1 wherein i is calculated fromthe following formula:

where:

V/x is the voltage stress (volts per centimeter) at which the surfacebreaks down; this means that surface discharge cannot take place at alower stress,

E is the voltage across the insulator,

L is the minimum distance along the insulator between its ends,

i, is the minimum current of the permitted discharge and is greater than7 mA,

1', is the current that normally flows through the surface of theinsulator when the insulator is dry, and

the surface conductance is calculated from the value of i,

using Ohms law.

3. An insulator in accordance with claim 1 wherein the surfaceelectrical conductance increases with increasing voltage stress.

a a a a a i

2. An insulator in accordance with claim 1 wherein ig is calculated fromthe following formula: where: V/x is the voltage stress (volts percentimeter) at which the surface breaks down; this means that surfacedischarge cannot take place at a lower stress, E is the voltage acrossthe insulator, L is the minimum distance along the insulator between itsends, ip is the minimum current of the permitted discharge and isgreater than 7 mA, ig is the current that normally flows through thesurface of the insulator when the insulator is dry, and the surfaceconductance is calculated from the value of ig using Ohms law.
 3. Aninsulator in accordance with claim 1 wherein the surface electricalconductance increases with increasing voltage stress.